The invention relates to molds for molding tread patterns for the treads of tires and in particular to molds for molding in a tread pattern motif at least one cutout, part of which is located beneath the running surface and opens on to at least one lateral face of said tread pattern motif.
In order to impart adhesion performance to a tread, it is known to form a tread pattern formed of motifs in relief on the surface of said tread which is intended to be in contact with the ground during travel. These motifs are defined by the cutouts (notches or grooves of a width of greater than 2 mm) which are produced by molding by means of elements in relief which are borne by a mold which is furthermore intended to vulcanize the rubber mixes comprising said tread. Furthermore, it is known to form, during the molding phase or alternatively by cutting-out after said molding phase, a plurality of incisions (cutouts of average width less than 2 mm) in the motifs in relief of the tread pattern so as to create a large number of rubber ridges to cut the film of water which may be present in the zone of contact.
Obtaining good adhesion performance of the tire on wet ground requires a very large number of cutouts (grooves and/or incisions) to be produced, which is incompatible with maintaining a sufficient level of rigidity of the motifs in relief of a tread pattern to withstand the forces exerted by the ground during contact. This is why it was proposed, in application WO 98/35842, producing a plurality of cutouts provided on the walls defining each of said cutouts of at least one rubber connecting element connecting said walls to each other. One embodiment has been proposed which consists of incorporating in the tire in the non-vulcanized state spacers which are provided with at least one hole for allowing the rubber mix to pass through during the molding operation and thus to form at least one connecting element. One disadvantage of this method resides in the fact that the incision after molding and vulcanization remains filled with the material forming the spacer. There is therefore no possibility of evacuating the water picked up during travel on wet ground.
In order to minimize the risks of formation of a fissure at the bottom of the incision, it is known, for example from U.S. Pat. No. 2,121,955, to mold a transverse channel beneath the running surface, this channel being located in the extension of each incision. One form of embodiment proposed consists in molding each channel by means of a needle during molding of the tire, which needle is introduced transversely and beneath the tread to be molded, and finally producing the incisions by connecting each incision to a channel.
Another form of embodiment consists in using molding elements formed of two assembled parts, one part molding an incision and another part molding a channel in the extension of the incision. However, in this latter case, the incision must itself open out laterally to permit demolding of these molding elements.
There has been discovered a need to be able to provide a means for molding a tread pattern in which at least some motifs in relief are defined by lateral faces and provided with cutouts (grooves and/or incisions) which open at least partially on to at least one of said lateral faces to enable said cutouts to communicate with at least one cutout adjacent said motifs, said cutouts furthermore having their main facing walls connected by at least one connecting element of rubber mix.
One object of the present invention is to propose a mold for molding a tire comprising a tread provided with a tread pattern formed of motifs in relief defined by cutouts, each motif in relief comprising at least two lateral faces spaced apart in the axial direction of the tire and a contact face forming part of the running surface of said tire.
At least one motif in relief comprises a cutout which opens both on to the running surface of the tread when new and on to at least one of the lateral faces of said motif, the main facing walls of this cutout being connected by at least one connecting element of rubber mix, the contour of the section of intersection of which with at least one of the main facing walls does not touch the bottom of the cutout.
The tire mold according to the invention, which is intended to mold a tire provided with a tread of rubber mix and having a running surface and two lateral faces axially defining said tread, comprises in the molding configuration:
a peripheral ring formed of sectors having a molding surface intended to mold at least the running surface of the tread, each sector being able to be moved in a movement towards or away from the axis of the mold,
two shells concentric to the peripheral ring, each having a molding surface intended to mold a lateral face of the tread, these shells being able to be moved in a movement in the direction of the axis of the mold which tends to move them towards or away from each other,
at least one first element in relief which projects from the molding surface of at least one of said shells for molding a channel beneath the running surface of the tread,
at least one second element in relief which projects from the molding surface of a sector of the ring for molding a cutout in the tread.
This mold, in the molding position, is characterized in that at least one first element in relief borne by a shell cooperates with at least one second element in relief borne by a sector to form a molding element intended to mold in at least one motif in relief of the tread pattern a cutout opening on to the running surface of the tread, said cutout being extended to the inside of the tread by a channel opening on to at least one lateral face of the motif in relief in question, said first and second elements in relief being functionally arranged to define at least one orifice which is entirely defined by said first and second elements in relief. Each orifice thus formed by the mold in the molding configuration is intended to allow the rubber mix to pass during molding for molding a connecting element connecting the main walls defining the cutout molded with said molding element.
A mold according to the invention permits easy molding of a cutout which opens on to the running surface of a tread when new and, for example, on to the lateral faces of said tread or alternatively the molding of a cutout on a motif in relief which does not have a common lateral face with the lateral faces of the tread (in this case the cutout opens beneath the running surface on at least one of the lateral faces of said motif in relief to cause said cutout to communicate with another cutout adjacent said motif).
In practice, the demolding, that is to say, the operation consisting of extracting the molded tread from the mold after vulcanization, does not cause any particular problems. Advantageously, the molds and the sectors may be opened simultaneously without any risk of breaking the molded connecting elements. For certain geometries of the first elements in relief, preferably the molding is carried out in two stages, in a first stage by radially moving apart the various sectors bearing the second elements in relief and in a second stage by moving the shells apart from one another in the axial direction in order to extract the first elements in relief borne by said molds in order to terminate the demolding of the tread (thus these first elements are prevented from being inhibited in their movement by the second elements).
The first elements in relief may have geometries which are rectilinear or alternatively curved, depending on the desired shape of the channel, for example in order better to follow the transverse curvature of the reinforcement armatures of the ring of a tire. In the case of a first element in relief having a curved geometry of high curvature (that is to say one which does not extend essentially in the transverse direction of the tread), the mold bearing said element in relief may be advantageously mounted to pivot about its axis of symmetry in order to facilitate extraction of said element in relief.
Another form of mounting the first elements in relief on one of the shells consists in providing an appropriate number of holes on this shell, this number corresponding to the number of first elements, for introducing these first elements so that they project from the molding surface of said shell.
One and the same first element in relief borne by a shell may, furthermore, be provided to cooperate with a plurality of second elements in relief borne by one or more ring sectors, said second elements being functionally arranged with said first element to mold cutouts, the walls of which are connected by at least one rubber connecting element.
What has been described above for a tire mold applies similarly to the molding of a tread alone, whether or not this tread be in the form of a closed ring or alternatively that of a flat tread (the direction of its width corresponding to the transverse direction once said tread has been put in place on a tire).
To this end, there is proposed a mold for molding a tread comprising an upper outer face intended to form a running surface extended laterally by lateral faces, these lateral faces furthermore being connected by an inner face. This mold comprises, in the molding position:
at least two mold parts defining a closed volume corresponding to the volume of the tread to be molded, said mold parts comprising molding surfaces molding the outer, inner and lateral faces of the tread,
at least one first element in relief, intended to mold a channel opening on to at least one lateral face of the tread, cooperating with at least one second element in relief intended to mold an incision which opens on to at least one of the inner or outer surfaces of the tread, for molding a single cutout, said first and second elements being functionally arranged to define in total at least one orifice intended to allow the rubber mix forming the tread to pass through during the operation of molding said tread.
One variant of this mold consists in that a first element in relief molding a channel opening at least on to one lateral face of a tread cooperates with at least one second element in relief molding an incision which opens on to the outer surface and at least one third element in relief molding an incision which opens on to the inner surface. In this variant, the incisions molded by the second and third elements in relief are connected together by means of the channel molded by the first element in relief. In this case in point, at least one of the second or third elements in relief is functionally arranged with the first element in relief to define in total at least one orifice intended to allow the rubber mix to pass to form a connecting bridge connecting the walls of the cutout thus molded.
The invention will now be described with reference to the drawing of two embodiments, which are given solely by way of examples.